1. Field of the Invention
This invention relates generally to power transmission component testing and specifically to a method and apparatus for dynamically loading a locked gear train testing rig to produce required load and speed conditions.
2. Description of the Prior Art
In the power transmission manufacturing industry, it is desirable to test transmission components such as gears, shafts, couplings, bearings, and the like at their design and maximum speed and power ratings for quality assurance purposes. Testing transmission components designed for low power applications is straight-forward, because there is an ample supply of power sources and loads available to build a complete power train testing setup. As the power levels for testing increase, both the prime movers (e.g. large electric motors, steam or gas turbines) and loads (e.g. electrical or hydraulic dynamometers) become more costly. To eliminate the need for high power sources, the power transmission manufacturing industry developed and use the locked gear train, as illustrated in FIG. 1 of the attached drawings, to test transmission components at high power and torque.
Referring to FIG. 1, a locked gear train 10 is a system including two gear units 13A, 13B of the same ratio and shaft separation. Each gear unit includes a pinion 14 with pinion shaft 16, a gear 18 with gear shaft 20, two bearings 22 per shaft, a housing 23 that supports the bearings and encloses the pinion and gear assemblies, and a lubrication system. One coupling 25 each is attached to the two pinion shafts 16, connecting a pinion connecting shaft 26 therebetween. Likewise, one coupling 24 each is attached to the two gear shafts 20, connecting a gear connecting shaft 28 therebetween. The couplings 24, 25 can be gear type, diaphragm or disk type, rigid flanged, etc. Respective gear and pinion helices are shown as having opposite hand angles, but prior art arrangements as of FIG. 1 also apply to identical hand gears of single identical sign.
A locked gear train is used to test power transmission components, because the pinions, gears, shafts, couplings, and bearings can be operated at high power levels without requiring a large power source. The theory of operation is based on torsional spring energy stored in the locked gear train. In a perfectly aligned locked train, the pinions and gears mesh easily with no undue static force from one gear element to another; the gear elements turn freely with little friction. There is no spring energy stored in an uncoupled or unloaded locked train. In a coupled or loaded locked train, when angular deflection is introduced, a static torque is induced. The active gear tooth surfaces of the pinions and gears abut tightly against each other, and the rolling friction of the system is increased. There are equal and opposite forces acting at the active pinion and gear tooth surfaces resisting the angular deflection, and there are reaction forces at the bearings holding the locked train in static equilibrium. A loaded locked train stores spring energy as torsional deflection in the locked train components.
In the prior art, static torque is intentionally induced by assembling the locked train with an angular misalignment, usually at one of the couplings. The angular misalignment must be compensated for by the introduction of a corrective angular deflection which imparts the torque to the locked train. This arrangement is referred to as a pre-torqued locked gear train. An attached motor 52 is used to rotate the pre-torqued locked gear train. When rotated, power level at the pinions, gears, shafts and couplings of the pre-torqued locked gear train is much greater than the power output of the motor, because locked gear train components are subjected to pre-torque. The motor is not required to supply much torque to achieve the high power levels required for testing. The power level at each locked gear train component is the product of the pre-torque value and the angular velocity. With a unity unit conversion factor included, the power-torque-speed relationship is:
P =cNTwhereP =power (H.P.)N =speed (rpm)T =torque (ft. lbs.)c =conversion factor (5252 H.P./(rpm ft. lbs.)).
The power levels within the rotating pre-torqued gear train can far exceed the power required of the motor which rotates the locked train. In other words, the power levels at each component of the locked train is the result of the artificially induced pre-torque. This resulting power level is transmitted in a closed loop within the locked train. The motor, existing outside the locked train, is decoupled from the resulting power level of the locked gear train. The power required to rotate the pre-torqued locked gear train is only that which is required to overcome inertia and friction. The actual power required to run the test set-up for a given speed/power test is only a fraction (typically five percent or less) of that resulting power level within the locked gear train.
A power transmission component must be tested at a given power and speed. The component may be a gear, pinion, shaft, coupling, bearing, etc. The component is assembled in the locked train arrangement. Next, the required pre-torque is calculated from the power-torque-speed relationship:   T  =      P    cN  where
T =torque (ft lbs.)P =power (H.P.)N =speed (rpm)c =conversion factor (5252 H.P./(rpm ft. lbs.))
The required pre-torque is then applied to the locked gear train, generally by uncoupling one of the couplings 24, applying torque to a main gear connecting shaft by rotating it an angular deflection of Θ radians while holding the facing main gear shaft stationary, and then re-coupling to lock in the pre-torque. This procedure builds up torsional spring force in the locked train. The torque-angular deflection relationship is expressed by Hooke's law as:                Θ=−T/k        where        
Θ =angular deflection (radian)T =torque (ft. lbs)k =spring constant (ft. lbs./radian)
The spring constant k is for the entire locked train and can be determined by appropriate modeling techniques or empirical data from experimentation. The locked train, containing the component to be tested, is then rotated at speed N, while quality control parameters are monitored.
Static torquing of the locked train has inherent difficulties. Only one torque level can be applied without uncoupling, re-torquing, and re-coupling. For each power-speed datum required, the time consuming procedure must be repeated. The static torquing method also risks damage to the bearings from static loading, and it is dangerous because of the safety hazard to test personnel when coupling and uncoupling shafts with high torsional spring energy stored in the gears.
Dynamic torquing methods have been developed which alleviate the disadvantages of the static torquing method described above. With dynamic torquing methods, the gear train is brought up to speed under no load conditions, so that the bearings can develop full film before the load is dynamically applied.
One method is to have a rotary actuator, which utilizes hydraulic force to rotate one shaft in relation to the other, applying the necessary toque. Rotary actuators have speed and torque limitations and require balancing and maintenance.
Another method employs a sliding gear on a spline, but this method has the problem of the spline seizing or sticking under high load, causing discontinuous load application, i.e. skipping or stepping.